The invention relates to a method of etching an epoxy resin having a high backetching rate, in particular when etching bores with printed circuit boards, when using stable, strong, basic alkaline permanganate etching solution.
When manufacturing multilayer circuits, it is necessary to carry out a preliminary process to remove resin smears resulting from forming a bore. The removal of these smears should insure a reliable contact of conducting multilayers-copper inner layers with a chemically formed copper coating. The purification of the bore effected by removal of epoxide, with simultaneous roughening of the outer surface of the substrate, insures in addition a better adhesion which, with a deenergized metal coating or coating obtained by direct electrolysis. This is important above all because of the thermal load which the circuit board is exposed to during subsequent soldering.
It is known to carry out the above-mentioned preliminary process with alkaline potassium or sodium permanganate solutions, when, as a rule, before oxidation, conditioning with an organic solvent and/or a surface-active agent is carried out, which however requires significant treatment time. Because, in most cases, only small backetching rates are achieved, it has already been proposed to increase the backetching rates, for example, by:
using ultrasound; PA1 increase of the temperature; PA1 increase of the concentration of permanganate by using sodium permanganate; PA1 increase of OH-ion concentration.
This, however, results either in the damage of the base material or in a strong destabilization of the permanganate system.